Conventionally, with a fuel reforming apparatus for producing reformed gas (gas containing 40 volume % or more (dry base) of hydrogen) with using fossil fuel such as natural gas as raw material, the raw material was desulfurized and steam-reformed through a desulfurizer and a steam reformer disposed one after another, thereby to obtain the reformed gas containing hydrogen as the major component thereof and carbon monoxide, carbon dioxide (CO2), water (H2O), etc. Further, with a fuel reforming apparatus using an alcohol such as methanol as the raw material, the apparatus includes a methanol reformer incorporating a methanol reforming catalyst, thereby to obtain, from methanol, a reformed gas containing hydrogen as the major component thereof and carbon monoxide, carbon dioxide, water, etc.
Here, with a fuel reforming apparatus for making a reformed gas for use in a phosphoric acid fuel cell, it is known that the electrode catalyst of the fuel cell is poisoned by the presence of carbon monoxide. Therefore, the gas containing hydrogen as the major component thereof was introduced to a carbon monoxide shift converter for converting carbon monoxide into carbon dioxide through a carbon monoxide shift reaction, thereby to obtain a reformed gas with the carbon monoxide concentration in the gas being lower than a predetermined value (e.g. 0.5%).
However, in the case of a fuel reforming apparatus for producing a reformed gas for use in a polymer electrolyte fuel cell, since this polymer electrolyte fuel cell operates at a low temperature of about 80° C., its electrode catalyst will be poisoned even if just a trace amount of carbon monoxide is present. Therefore, it is necessary to further reduce carbon monoxide to be contained in the reformed gas. So, on the downstream of the carbon monoxide shift converter, there was provided a carbon monoxide removal reactor incorporating a carbon monoxide removal catalyst for removing carbon monoxide. With this, the reformed gas treated by the carbon monoxide shift converter was introduced, with addition thereto of an oxidizer such as air, to the carbon monoxide removal reactor, so that carbon monoxide was oxidized into carbon dioxide in the presence of this carbon monoxide removal catalyst, whereby a reformed gas with reduced carbon monoxide concentration lower than a predetermined concentration (e.g. 100 ppm or lower) was obtained.
The carbon monoxide removal reactor includes, in its casing, an accommodating portion for accommodating a catalyst layer formed of a carbon monoxide removal catalyst such as ruthenium (Ru), rhodium (Rh), platinum (Pt), palladium (Pd) or the like supported on a support made of e.g. alumina. In operation, gas (reactant gas) containing the reformed gas and an oxidizer such as air added thereto is introduced through a gas inlet to the catalyst layer in the accommodating portion so as to come into contact with the carbon monoxide removal catalyst, whereby the carbon monoxide in the reformed gas is converted into carbon dioxide. Then, the reactant gas whose carbon monoxide content has been reduced by the passage through the catalyst layer is discharged from a gas outlet formed through the casing. Further, with the carbon monoxide removal catalyst, the reaction for oxidizing the carbon monoxide is selectively promoted when the temperature of the catalyst layer is about 80 to 200° C. Therefore, the convention provides a temperature adjusting means (heater, cooler or the like) to the casing so as to maintain the catalyst layer at such temperature range.
Incidentally, as the material forming the fuel reforming system, the convention has employed mainly stainless steel, which is an iron containing material, with consideration to various factors such as corrosion resistance, heat-resistance, strength, workability, cost, etc.